It happened at Brandywine Heights High School in Mertztown Wednesday morning.
The school superintendent had decided students who wanted to participate in the national event could attend a symposium on school safety in the auditorium, but not walk out like thousands of students across the country.
Students across region participate in National School Walkout
They were calling for stronger gun safety measures after the school shooting massacre in Parkland Florida.
The 21 students chose to violate that ruling and gathered by the flagpole. All of them were suspended for one day.
At the symposium, school officials shared a new security measure that would scan all visitors' driver's licenses to check for red flags in their backgrounds.
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